Antigenicity: The ability to combine/react specifically with the final products of the above responses (i.e., antibodies and/or cell-surface receptors).

All molecules that have the property of immunogenicity also have the property of antigenicity but Reverse not true. Remember: All Immunogen are Antigen but all Antigen are not Immunogen e.g. Hapten.

Hapten are antigenic but incapable by themselves of inducing a specific immune response, i.e., they lack immunogenicity

Epitopes

Immune cells do not interact with or recognize an entire immunogen instead they recognize discrete sites on the macromolecule which is called epitopes.

Epitopes are immunologically active regions of an immunogen that bind to antigen-specific membrane receptors on lymphocytes or to secreted antibodies.

B cells and T cells recognize different epitopes on the same antigenic molecule.

B cell bind epitopes that are present in highly accessible sites on the exposed surface of the immunogen.

T cells recognize only peptides combined with MHC molecules on the surface of APCs or altered self cells.

Hapten

A substance that is non-immunogenic but which can react with the products of a specific immune response.

Haptens are small molecules which could never induce an immune response when administered by themselves but which can when coupled to a carrier molecule.

Haptens have the property of antigenicity but not immunogenicity.

Immunoquiz-1:

The substances that are least immunogenic are

a) Proteins

b) Polysaccharides

c) Nucleic acids

d) None of the above

Factors influencing Immunogenicity

Nature of the Immunogen

Foreignness

Molecular size

Chemical composition and heterogeneity

Ability to be processed and presented with an MHC molecule on the surface of APC or altered self-cell

Biological system that the antigen encounters

Genotype of the recipient animal

Dosage and route of administration

Factors influencing Immunogenicity

Ⅰ. Foreignness

Must be recognized as non-self by the biological system

Degree of immunogenicity depends on the degree of foreignness i.e. The greater the phylogenetic distances between two species, the greater the structural (and therefore the antigenic) disparity between them.

e.g. If Bovine serum albumin is injected in Cow, Rabbit and Chicken, the order of Immunogenicity will be:

Cow < Rabbit < Chicken (least for cow and most for chicken)

This property is govern by: Tolerance to self (specific unresponsiveness to self antigens)

Concept of tolerance

During lymphocyte development immature lymphocyte are exposed to self-components.

Those that reacts with self antigen are killed (clonal deletion)

Antigens that have not been exposed to immature lymphocytes during this critical period may be later recognized as nonself, or foreign, by the immune system.

Polyclonally activate a large fraction (up to 25%) of the T cells, setting off massive immune response

In T dependent Antigen only a fraction of (1 in 106 -104) of the T cell population is able to recognize the antigen and become activated

Examples of superantigens include:

Staphylococcal enterotoxins

Staphylococcal toxic shock toxin (TSST-1)

Streptococcal pyrogenic exotoxins (exotoxin A and exotoxin B)

Heterophilic Antigen

1.An antigen that is possessed by a variety of different phylogenetically unrelated species. Antibodies induced by these antigens cross-react with individual heterophilic antigens.
This antigen type is involved in the pathogenesis of certain diseases, e.g. infectious mononucleosis, rheumatic fever, glomerulonephritis.

e.g. Antigen of Group A beta hemolytic streptococci (Streptococcus pyogenes) and antigens of the human myocardium are heterophilic

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Hello, thank you for visiting my blog. I am Tankeshwar Acharya. Blogging is my passion, I am working as a Asst. Professor and Microbiologist at Department of Microbiology and Immunology, Patan Academy of Health Sciences, Nepal.
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